Air cylinders are used in large variety of different machinery, mechanisms and devices for executing reciprocating linear motion of the weight attached to the cylinder piston rod.
To decelerate and stop moving weight without damage to the air cylinder and metal-to-metal banging, the following features are most commonly used: hydraulic shock absorbers, built-in resilient bumpers and air cushioning mechanisms, all of which have certain disadvantages.
Hydraulic shock absorbers make air cylinder systems substantially more bulky and complicated. Due to the problem of overheating, they are not suitable for high speed/high frequency applications.
Because oil cannot be compressed, there is significant mechanical impact when cylinder piston hits the hydraulic shock absorber rod at high velocity.
The built-in resilient bumpers absorb only limited amount of energy and usually bounce back after deceleration is completed.
The air cushioning mechanism's performance and problems associated with conventional air cylinders in general are discussed below:
FIG. 1 illustrates conventional air cylinder system comprising air cylinder 10, directional control valve 2, flow control valves with adjustable orifice 3 and 5 and check valves 4 and 6 permitting air flow in one direction only.
Air cylinder has air cushion mechanisms and consists of cylinder body 15 and piston 13 moving axially inside of this body; Piston is sealed against body inner surface by sealing structure 16 and is connected to the rod 14. Cylinder body ends are closed by the front-end head 17 and rear end head 18 with cavities c and sealing structures 9.
Each end head has an inlet/outlet port and adjustable needle valve 11 or 12.
Piston rod 14 protrudes through the front-end head and is supported by bearing 20 and is sealed off from the atmosphere by a sealing structure 19.
When directional control valve 2 is in the position shown, compressed air from compressed air source 1 through directional valve 2, check valve 4, inlet port e of front end head and cavity c enters into cylinder pressurized chamber A with the pressure P1 and starts to accelerate piston 13, rod 14 and attached weight in the direction of the arrow.
Air from cylinder chamber B through rear end head cavity c, outlet port d, flow control valve 5 and directional valve 2 is being discharged to the atmosphere.
As piston velocity increases, the flow control valve 5 resistance and subsequently chamber B air pressure P2 also increase.
As a result (see FIG. 1/3), at certain point of the piston stroke (so called equilibrium point) force of the air pressure P1 in chamber A applied to the piston in arrow direction becomes about equal to the friction force and force created by air pressure P2 in chamber B, acting in opposite direction.
From this point on, the piston moves with approximately uniform velocity (so called “flow controlled” motion) up to the moment when rear cushion spear 7 engages with sealing structure 9 (see FIG. 1/2).
After this, cavity c gets closed and compressed air from chamber B could be discharged into the atmosphere through the needle valve 11 orifice b only, with much higher resistance.
This leads to the surge of pressure P2, and, as a result, piston resistance force in chamber B becomes bigger than piston driving force in chamber A, which provides moving weight deceleration.
However the air cushion mechanism as described above, has a substantial problem:
The resistance of needle valve 11 orifice b also depends on piston velocity.
As piston velocity decreases along the deceleration path of the stroke, orifice b resistance and pressure P2 also decrease which seriously affects mechanism stoppage ability. In another words cushion mechanism deceleration force is not uniform and after initial peak surge diminishes very rapidly significantly limiting deceleration capacity.
In some cases (limited intake volume of compressed air or prolonged acceleration) pressure P2 could be much lower than P1, which will make air cushion mechanism even less effective.
In addition to the problem with air cushion mechanism, practically any conventional air cylinder system has more design-inherited problems:                100% of compressed air volume involved in producing air cylinder piston reciprocating motion is being discharged to the atmosphere, resulting in big energy (money) loss.        In many applications, after piston acceleration is completed and maximum velocity is reached, piston continues to move under high pressure in pressurized and discharged chambers, thus uselessly consuming extra volume of compressed air. This results in additional energy (money) loss and a large driving (propelling) force. Absorbing propelling force energy takes significant share of total absorbed energy which otherwise can be used more productively.        Air cylinder's ability to accelerate moving weight usually substantially exceeds cylinder stoppage capacity. To make them about equal, cylinder's accelerating ability is very often unnecessary reduced (by adjusting flow control valve on discharge line). This prevents cylinder from being used to its full potential. With higher stoppage capacity, the same air cylinder could have worked in much more powerful acceleration mode.        To start each and every piston stroke, air pressure in pressurized chamber must be built up again and again from atmospheric to the operational level. This is achieved by throttling all compressed air volume through the orifice of directional control valve, which, in turn, delays acceleration, increases stroke completion time and subsequently reduces cycling frequency thus making the cylinder less suitable for high speed, high cycling applications.        